POMCFs Photocatalysts Could Help Convert CO2 into CH4 in a Photoreduction System

Environmental issues and global warming are the results of excessive discharge of CO₂ produced from the constant burning of fossil fuels.

Changing surplus CO₂ into a serviceable energy product through artificial methods represents a significant pathway toward sustainable development. A viable method for the above-mentioned conversion is solar-driven photocatalytic reduction of CO₂ to carbon-neutral fuels like CH₄ and CO and/or value-added chemicals like CH₃OH and HCOOH.

By implementing this reaction, the greenhouse effect and also the energy concerns can be reduced concurrently. However, due to the high C=O bond cleavage enthalpy and intrinsically chemical inertness of the CO₂ molecule, the structural activation process of this molecule is especially challenging.

Hence, to overcome the extremely negative equilibrium potential (against NHE) for thermodynamically adverse CO₂, intermediate, proton-assisted multiple electron reductive products such as chemicals and/or hydrocarbon are usually achieved to reduce the activation energy of the photocatalytic conversion of CO₂.

Despite that fact, the creation of high-order electron and proton transferring products still has to overcome a significant kinetic obstacle, and the competitive evolution of H₂ further escalates the challenge of achieving the targeted product selectively.

For example, CH₄ photosynthesis has posed a major challenge because the achievement of eight-electron transport process needs the photocatalyst to provide both adequate electrons theoretically and potent reducing capability. CH₄ is considered the most valuable and desirable hydrocarbon fuel in a photoreaction system.

Scientists perceived that polyoxometalate (POM)-based coordination frameworks, or POMCFs for short, with favorable catalytic performance and familiar structural stability, could be more advantageous to carry out the photocatalytic reduction of CO₂ owing to the synergistic effect resulting from the incorporation of MCF and POM.

Specifically, the Zn-ε-Keggin cluster of PMo12 “electron sponges” family, that also includes eight MoV atoms, can act as a powerful reductive component and contribute eight electrons hypothetically.

Four-trapped Zn(II) located in ε-Keggin (PMo12) also forms the Zn-ε-Keggin—a tetrahedral node. When compared to a majority of anionic POMs, the ε-Keggin altered with metal Zn turns out to be a cationic cluster, which is conducive for coordination with organic ligands.

As a result, if the porphyrin derivative and reductive POM cluster can be used to develop POMCF, having the visible-light harvesting as well as photo-excited electron migration, that would serve as an excellent method towards selective photoreduction of CO₂ to multi-electron reductive products.

Hence, two POMCFs—NNU-14 and NNU-13—were ultimately developed. These were produced with visible-light responsive TCPP linker and reductive Zn-ε-Keggin cluster. The POMCFs have high photocatalytic CH₄ activity and selectivity (greater than 96%) that have considerably exceeded a number of MCF-based photocatalysts.

Hypothetical calculations showed that the photo-generated carriers of CB and VB are largely distributed on Zn-ε-Keggin cluster and TCPP group, respectively. It was observed that the photo-excited electrons flow more easily into the POM port through efficient inter-coupling between the TCPP linker and reductive Zn-ε-Keggin unit.

It was also observed that the introduction of the POM building blocks with strong reducing ability imparts both NNU-14 and NNU-13 with conducive structural rigidity. It also eases the photocatalytic CH₄ selectivity by hypothetically delivering sufficient numbers of electrons to achieve the eight-electron reduction of the CO₂ molecule.

This viable technique method, organizing potent reducing component into visible-light sensitized photocatalyst architecture, are expected to trigger research enthusiasm towards the development of efficient POMCFs photocatalysts for highly selective reduction of CO₂ to CH₄ or other similar high-valued hydrocarbons.

Source: http://www.scichina.com/